Metal nanowires have attracted considerable interest in the past decade due to their remarkable transport and structural properties. Gold and silver nanowires were observed to form spontaneously under electron irradiation, and appear to be stable. Even the thinnest gold wires, essentially chains of atoms, have been observed to exhibit lifetimes of the order of seconds at room temperature. Metal nanowires exhibit striking correlations between their stability and electrical conductance. Although not limiting the scope of the invention, the stability of filamentary structures can be explained by electron-shell effects. Because most of the atoms of a nanowire are at the surface, with low coordination numbers, metal nanowires behave essentially like fluids. Classically, the Rayleigh instability would break up any wire whose length exceeds its circumference. Nevertheless, nanowires violating the Rayleigh criterion have been observed. The instability is suppressed through quantum effects, with stabilization occurring through the nanowire's electronic shell structure. A quantum linear stability analysis showed the existence of “islands of stability” for discrete intervals of the radius R. These correspond to conductance “magic numbers” that agree with those observed in experiments. For low enough temperatures, there remain finite regions of R stable against long-wavelength perturbations. Therefore, stable wires exist only in the vicinity of certain “magic radii” and consequently at quantized conductance values G that are integer multiples of the conductance quantum G0=2e2/h. However, this linear stability analysis ignores large thermal fluctuations that can lead to breakup of the wire. Nanowire lifetimes are inferred from conductance histograms which are compiled by cycling a mechanically controllable break junction thousands of times. Prior art studies indicate that conductance peaks disappear above fairly well-defined temperatures.
The lifetimes of these nanowires have been studied in the art by using conventional techniques. By modeling thermal fluctuations through stochastic Ginzburg-Landau classical field theories, a self-consistent approach has been constructed to provide an explanation of the fluctuation-induced “necking” of nanowires that is in good agreement with prior art experiments. This theory indicates that passivated noble metal nanowires are sufficiently stable at room temperature to serve as interconnects between nanoscale circuit elements.
On the experimental side, nanowires with diameters less than a nanometer have been directly observed using transmission electron microscopes (TEM) to remain stable under low beam intensities below 5 A/cm2 for the duration of observation. Stochastic switching between different conductance values has been observed in contacts made using mechanically controllable break junctions, while controllable switching has been achieved recently using electromigration to grow or shrink a nanobridge between two wires. A structural thinning process of the nanowire similar to the one described by the theory of Biirki et al. has been observed to take place for gold nanowires in TEM experiments. The nanowire was observed to thin step by step via a process where a structural step (corresponding to a change in radius of the order of one atomic diameter) forms at one end of the wire and subsequently propagates along the wire.
Nanowires suitable for the proposed device, i.e. with conductance between a few and a hundred conductance quanta, and lengths below or around a few nanometers have been realized experimentally using various techniques:
a. Scanning Tunnelling Microscopy (STM)
In this technique, a STM tip is crashed into a metal sample, thus forming a nanocontact. The size, and thus conductance, of the metal contact can then be adjusted to a predefined value by adapting the tip-sample distance. This technique has been shown to form relatively short wires down to atomically thin cross sections. However, it is sensitive to drift in the STM tip position, as well as to external vibrations. Furthermore, it is limited to metals which can be used to fabricate a sharp tip (Au, W, and other metals known in the art to be fabricable). However, a STM tip could be used to control strain on a longer, preformed nanowire, thus inducing conductance changes.
b. Mechanically Controllable Break-Junction (MCBJ)
With this technique, a macroscopic wire is glued to a support, itself mounted on a three-point bending device. A notch in the macroscopic wire provides a weak point where the wire breaks upon bending of the substrate. A nanocontact is formed, and its size can be controlled by adjusting the bending of the substrate. As for the STM technique, relatively short wires are formed, but the setup is more stable (i.e. less sensitive to external vibrations) and more versatile, as essentially any metal can be used. However, the need for such a three-point bending device, while allowing control of the strain applied to a nanowire, makes it difficult to have a fully nano-sized device.
c. Thin-Film Transmission Electron Microscopy (TEM)
In contrast to the previous techniques, long wires can be formed by burning holes into a thin metallic film deposited on a substrate, using a strong electron irradiation. When two holes come close together, the irradiation intensity is decreased, switching the TEM to imaging mode, and a thin bridge is left, and is seen to self-assemble into a long, almost perfectly cylindrical wire. The nanowire thus formed is found to be suspended above the substrate, and has typically a diameter between half and a few nanometers, and a length of several nanometers. There is up to now little control on the final size of the wire, which is seen to be stable for seconds or longer at room temperature. As an alternative, a scanning electron microscope (SEM) could be used rather than TEM.
d. Electromigration and Electrochemical Fabrication.
These two methods have been developed recently, and the wire size can be controlled in the former case by electromigration due to a voltage across the wire, or in the latter case by an electrochemical potential which controls etching from, or deposition on the wire through an electrolyte, while the presence of the electrolyte has been shown to have little influence on the stability and transport properties of the wire. Both methods have been shown to allow good control of the conductance of the wire. The electromigration technique has been generalized to create several contacts of similar size in parallel.
In order to change a nanowire configuration, several methods have been investigated: raising the temperature, applying strain and shortening the wire. For the purposes of a useful nanoscale device, these prior art methods are unsatisfactory for various reasons, principally having to do either with nonoptimal operating conditions (temperature), or inability to implement these controls on the nanoscale (strain). However, none of the prior art methodologies described hereinbefore enables creation of stable nanowires which can be rapidly and controllably switched between different conductance states.